The present invention relates to a sampling device used to sample ambient atmospheres.
It is known to provide a sampling device in the form of a tube containing an adsorbing material. One end of the tube is normally closed by a tightly-fitting cap which is temporarily removed to sample the ambient atmosphere at a test location, and then replaced to prevent subsequent contamination of the sample.
Later, at an analysis site, the tightly-fitting cap is once again removed from the tube and is replaced by a more loosely-fitting cap. The sampling device is then loaded into a hopper for processing by an analytical instrument.
The loosely-fitting cap, whilst providing a worse seal than the tightly-fitting cap, is necessary to allow the cap to be removed by an automatic mechanism within the analytical instrument so that the contents of the sampling device may be analysed.
With the loosely-fitting cap removed, the contents of the sampling device are analysed by heating the adsorbing material to release therefrom any volatile organic compounds (VOCs) that may have been adsorbed during the sampling period, the desorbed VOCs being driven from the tube and into the analytical instrument by a flow of inert gas.
However, the process described above has a number of serious drawbacks. Firstly, the manual removal of the tightly-fitting cap is both difficult and time consuming. Secondly, as mentioned above, the loosely-fitting cap provides a poor seal which may allow a sample to become contaminated. Thirdly, the provision of an automatic cap-removal mechanism significantly increases the cost and complexity of the analytical instrument.
We have now devised an arrangement which overcomes the limitations of existing sampling devices.
According to a first aspect of the present invention, there is provided a sampling device containing an adsorbing material and fitted with a cap formed with at least one elongate passage through which gases may pass into and out of the device.
Preferably the cap is formed, at one end, with a socket to receive a connecting portion of the sampling device. Preferably the cap is a sliding fit over said connecting portion. Preferably the socket is provided internally with one or more xe2x80x98Oxe2x80x99-ring seals for embracing the outer surface of said connecting portion.
When tightly fitted to the sampling device at a test site following a sampling period, the elongate passage through the cap thus limits the rate at which gases may subsequently diffuse into or out of the device, but, at the same time, allows desorbed VOCs to be driven out of the device by a flow of gas for analysis.
Two major factors govern the optimum dimensions of the bore formed by the or each passage through the cap.
Firstly, the uptake of VOCs by the adsorbing material is governed by Fick""s Law, which states that the rate of uptake Q of a particular VOC is proportional to the cross sectional area A of the bore, and is also inversely proportional to length of the bore, more particularly   Q  =            DA      L        ⁢    t  
Where Q is the uptake quantity, D is the diffusion constant for a particular VOC, t is the duration of exposure, A is the cross-sectional area of the bore and L is the diffusion length.
Thus the rate of uptake may be controlled by varying either the diameter or the length of the bore.
However, there is a practical limit on the minimum diameter of the bore, which is determined by the maximum acceptable pressure difference p between the ends of the bore as inert gas is driven through the bore during analysis.
The pressure difference p between the ends of the bore is governed by Poiseuille""s equation which states that the pressure difference p is proportional to the length of the bore L and to the volume of gas flow V through the bore, and is also inversely proportional to the radius of the bore raised to the power 4, more specifically   V  =                              r          4                ⁢        π                              L          ⁢          8                ⁢                  xe2x80x83                ⁢        η              ⁢    p  
Where V is the volume of gas passing through the bore, r is the radius of the bore, L is the length of the bore, p is the pressure difference between the ends of the bore and xcex7 is the coefficient of viscosity of the gas.
So as not to exceed a maximum acceptable pressure drop, the minimum diameter of the bore must therefore be limited. Once this limit has been reached, any further reduction in the rate of uptake of VOCs can only be achieved by increasing the length of the bore.
One or more bores may be formed by respective passages which extend axially through the cap.
In this case, the or each passage is preferably provided by the bore of a respective capillary tube fitted into a bore through the cap. Preferably the passage has a length at least 10 times (more preferably 20 times) its width. For example, the passage may have a diameter of 0.01 inch (0.25 mm) and a length of 12 mm.
However, we have found that to provide the low rates of uptake required in most modern sampling applications, whilst maintaining a preferred pressure drop of around 1 psi for gas flows of up to 100 ml/min, a bore length far exceeding that achievable by means of an axial passage is typically required.
In order to provide a long bore within the dimensional confines of a conventional cap, at least one convoluted passage is preferably formed through the cap.
The or each convoluted passage may, for example, be provided by a respective helical tube disposed within a hollow compartment of the cap.
However, more preferably, the cap comprises a sleeve and a cylindrical insert, with one or other of the opposed surfaces of the sleeve and the insert being formed with at least one helical channel such that, when the insert is fitted into the sleeve, the or each helical channel forms a respective passage through which gases may pass into and out of the device.
According to a second aspect of the present invention, there is provided a cap comprising a sleeve and a cylindrical insert, one or other of the opposed surfaces of the sleeve and the insert being formed with at least one helical channel such that, when the insert is fitted into the sleeve, the or each helical channel forms a respective passage through which gases may pass into and out of the device.
According to a third aspect of the present invention, there is provided a method of analysing a sample held within sampling device, the method comprising the steps of fitting at least one cap of the type defined above to the sampling device following a sampling period, and offering the sampling device to an analytical instrument which provides a flow of gas through the sampling device to drive said sample out of the device through the cap.
Preferably two caps of the type defined above are fitted to the sampling device with said flow of gas being introduced into the device through the passage or passages formed in one of the caps to drive said sample out of the device through the passage or passages formed in the other cap.
According to a fourth aspect of the present invention, there is provided a sampling device having a tubular opening and a cylindrical insert, one or other of the opposed surfaces of the tubular opening and the insert being formed with at least one helical channel such that, when the insert is inserted into the opening, the or each channel forms a respective passage through which gases may pass into and out of the device.
According to a fifth aspect of the present invention, there is provided a method of analysing a sample held within a sampling device having a tubular opening, the method comprising the steps of fitting a tubular insert into the opening following a sampling period, and offering the sampling device to an analytical instrument which provides a flow of gas through the sampling device to drive said sample out of the device through at least one passage formed by a respective helical channel formed in one or other of the opposed surfaces of the tubular opening and the insert.
Preferably the device comprises two openings, each fitted with a respective insert, said flow of gas being introduced into the device through the passage or passages formed in one of the openings to drive said sample out of the device through the passage or passages formed in the other opening.